Radio-frequency identification (RFID) is an established technology for communicating with small electronic devices (“tags”) that can be attached to pallets, packages, or product instances. RFID tags can include passive circuits in an integrated circuit (IC) that respond to a radio signal with stored identification or other data. The radio signal is provided by a “reader” (or “interrogator”) that commands the tag to transmit its stored data. U.S. Patent Publication No. 2008/0204238 describes a variety of RFID-enabled devices. In this publication, the term “reader” refers to any electronic device capable of communicating with an information-storage device. The term “downlink” refers to communications from a reader to an information storage device, for example an RFID tag. The term “uplink” refers to communications from an information-storage device, for example an RFID tag to a reader.
RFID devices can be used for monitoring purposes, e.g., as disclosed in U.S. Pat. No. 7,268,680. This patent describes a tag unit having a transmitting unit coupled to wearable electronic banding material. An RFID unit with a writeable memory is coupled to the transmitting unit. The band can include one or more conductors (which can be an antenna) that complete an electronic circuit. A layer of the band can include the RFID tag IC. The RFID tag can be read to determine that it is operational. The tag can also return data indicating whether the band is still connected to the tag IC.
RFID devices can also be used for sensing. U.S. Patent Publication No. 2009/0058667 by Dixon et al. describes RFID tags connected to sensors to remotely monitor tire pressure. In this scheme, the RFID circuitry communicates with a sensor external to itself. This requires purchasing the separate sensor and connecting it to the RFID tag, which adds to the cost of the tag.
U.S. Pat. No. 6,970,731 to Jayaraman et al. describes sensors for monitoring vital signs, and specifically electrical impulses produced by a living body. However, this scheme is only useful for systems that monitor an electrical impulse. There is a need to monitor other conditions besides electrical signals.
Similarly, U.S. Pat. No. 7,351,191 to Mitchell et al. describes a sensor using multiple conductors to detect distortions of an applied electric field between the conductors. This is used to detect presence of a person in a space, e.g., the seat of a car. Mitchell et al. describe conductors embedded into a car seat assembly and surrounded with a substantially liquid-impervious but vapor-permeable material to permit using a separate temperature or humidity sensor to sense the temperature or humidity in the car and compensate for the sensed temperature or humidity when interpreting measured electric-field data. However, this scheme is also only useful for detecting objects that interact sufficiently with an electric field.